1. Field of the Invention
This invention relates to an apparatus for converting gray levels of an image, a method thereof, a program storage device thereof, and an infrared camera, and especially relates to an apparatus for converting gray levels of an image with multiple gray levels into fewer gray levels, a method thereof, a program storage device thereof, and an infrared camera to which the present invention is applied.
2. Description of the Related Art
There are images with multi-gray levels, for example 4096 gray levels to get a high resolution (which corresponds to 12 bit resolution), such as an infrared image taken by an infrared camera, a medical image taken by a computer tomography camera, etc.
Gray levels of a generally used display unit, however, are less than 256 (which corresponds to 8 bit resolution), and a gray level conversion is required to display an image with multiple gray levels on a generally used display unit.
The following problems may be caused when an original image is converted using a simple linear converting method.
For example, in an original image taken by an infrared camera, the intensity of objects with relatively high temperature (for example, people) is high, and the intensity of other objects with relatively low temperature (for example, a background) is low. When a simple linear conversion is applied to the image, the intensity of objects with relatively high temperature becomes high but lacks fine gradation. The intensity of objects with relatively low temperature becomes low but lacks a fine gradation.
Accordingly, the object of the present invention is to provide an apparatus for converting gray levels into fewer gray levels without missing a local gradation, a method thereof, a program storage device thereof, and an infrared camera to which the present invention is applied.
According to one aspect of the present invention, there is provided an apparatus for dividing an original image with multiple gray levels into the low-frequency component which represents a broad gradation of the original image and the high-frequency component which represents a local gradation of the original image; and changing the dividing characteristic used in said dividing means in accordance with an edge strength defined as a difference between the gray levels of neighbouring pixels.
An apparatus according to the first invention, can prevent a local gradation from being lost from the low-frequency component.
According to another aspect of the present invention, there is provided an apparatus further comprising a means for compressing gray levels of the low-frequency component divided by said dividing means into fewer gray levels, and a means for composing the compressed low frequency component with the high-frequency component.
An apparatus according to the second invention can prevent a local gradation from being lost from the low frequency component when the image with multi gray levels is displayed on a generally used display unit with fewer gray levels.
According to another aspect of the present invention, there is provided an apparatus, wherein said dividing means extracts a low frequency component from an original image with a dividing characteristic which is modified in accordance with an edge strength detected by an edge strength detecting means.
An apparatus according to the third invention adds edges having large gray level difference into the low frequency component.
According to another aspect of the present invention, there is provided an apparatus comprised a means for dividing pixels in an original image into an edge group which consists of pixels within a fixed width on both sides of an edge, and a non-edge group which consists of pixels except pixels in said edge group; and a means for extracting a low frequency component using an edge preserving smoothing filter for pixels in said edge group and using a non edge preserving smoothing filter for pixels in said non-edge group.
An apparatus according to the fourth invention, does not need to determine which one of said edge preserving smoothing filter or said non-edge preserving smoothing filter, is applied for extracting the low frequency component for every pixel, because pixels of the original image are previously divided into the two groups before extraction.
According to another aspect of the present invention, there is provided an apparatus which increases the size of a smoothing filter as edge-strength increases.
An apparatus according to the fifth invention extracts a low frequency component using a small size filter for the pixels in the edge group in order to keep edges, and using a large size filter for the pixels in the non-edge group.
According to another aspect of the present invention, there is provided an apparatus comprised a means for dividing pixels in an original image into an edge group which consists of pixels within a fixed width on both sides of edges, and a non-edge group which consists of pixels other than pixels in said edge group; and a means for extracting a low frequency component using a small size filter for pixels in said edge group and using a large size filter for pixels in said non-edge group.
An apparatus according to the sixth invention, does not need to determine which one of said small size filter or said large size filter is applied for extracting the low frequency component every pixel, because pixels of the original image are previously divided into two groups.
According to another aspect of the present invention, there is provided an apparatus comprised a means for dividing an original image into a low-frequency component and a high frequency component; a means for compressing gray levels of said low frequency component into fewer gray levels; a means for emphasizing said high frequency component; and a means for displaying an image composed from said compressed low frequency component and said emphasized high frequency component.
An apparatus according to the seventh invention, can display an image with multi-gray levels in a low resolution monitor without losing fine gradation.
According to another aspect of the present invention, there is provided a program storage device for storing a program executable to perform any one of the above-mentioned inventions.
According to further aspect of the present invention, there is provided an infrared camera to which the present invention is applied.